1. Technical Field
The present invention relates to a charged particle beam apparatus, a method of adjusting astigmatism using same and a method of manufacturing a semiconductor device using same for observing a surface of a sample with a high throughput at a high reliability to test and estimate the structure of a sample surface and an electric conductive condition (charged particle beam testing) by irradiating a surface of a sample with a charged particle beam, the sample being a wafer or a mask including patterns having a minimum line width of less than 0.1μ.
2. Background Technology
Japanese Patent Laid-open No. 2001-22986 describes an apparatus and method for observing and estimating a sample by irradiating the sample with an electron beam to detect secondary electrons, reflected electrons or backscattered electrons emitted from the sample. Japanese Patent Laid-open No. H05-258703 describes an electron apparatus wherein an electron beam is irradiated to a sample surface to detect secondary electrons emitted from the sample surface so as to combine an image from the detection results to obtain information about the sample surface.
In such a sample surface observing/estimating apparatus, the adjustment of astigmatism is essential to observe the surface at a high power. This is because an image is blurred as a result of an electron beam being deformed elliptically to either direction of rotation after the electron beam passes through an aperture, causing a longitudinal direction to deviate from a spot. In order to correct such a blurred image, it is necessary to apply an electric field or a magnetic field by a lens having eight to twelve or more poles to make the longitudinal direction of the electron beam narrower to form a spot-like electron beam. For example, Japanese Patent Laid-open No. H10-247466 describes a method of adjusting astigmatism using a magnetic field.
More specifically, as shown in FIG. 1(A), if an electron beam is deformed elliptically and a cross section thereof on a sample surface is deformed to an elongated shape in a direction of azimuth angle θ, it is possible to adjust the cross section of the electron beam to be like a spot by assigning suitable voltages to a pair of facing electrodes R1 and R2 positioned in the direction of azimuth angle θ, as shown in FIG. 1(B). Consequently, if, as shown in FIG. 2, it is possible to set the longitudinal direction θ of the electron beam to 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315°, it is possible to change, for example, an elliptically deformed cross section of the electron beam to a beam having a circle or a spot-like cross section, by disposing a pair of opposing electrodes on a line at any one of the above angles to optimize voltages to be applied to the pair of electrodes facing on the line which corresponds to the cross section to be adjusted.
In order to set azimuth angle θ and voltage V to be applied to the electrodes under optimum conditions, in the prior art, an applied voltage R has been adjusted such that a radial or ring-shaped pattern existing in a test pattern becomes sharp in all azimuth directions while observing the pattern. For example, Japanese Patent Laid-open No. H10-247466 discloses an SEM in which astigmatism is corrected by using a circular pattern.
However, a conventional algorithm for automatically adjusting astigmatism is disadvantageous because it is complex and difficult to understand. This is because it is impossible to extract an azimuth angle at which a pattern is blurred due to astigmatism, by using an autofocus function. Further, the test pattern must be observed previously when various wafers are finely adjusted. There is such a problem that astigmatism adjustment for every wafer is impossible if there is no test pattern on a wafer to be tested.
On the other hand, a relationship between primary electron beam irradiating energy and an efficiency of emission σ of secondary electrons is such as shown in FIG. 3. In a range where energy of irradiated primary electron beam is equal to or more than about 50 eV and equal to or less than 1500-2000 eV, efficiency of emission a of secondary electrons is equal to or more than one, and more secondary electrons than an incident electron beam is emitted. As a result, a surface of an insulating material is charge up positively. However, if the primary electron beam energy is above or below the above-described range, efficiency of emission σ becomes equal to or less than one, and the surface of the insulating material is charged negatively. There is such a problem that, if positive or negative charge up becomes large, an image formed from the secondary electrons for observation and estimation begins to be distorted, resulting in failure in acquisition of accurate information about the sample surface.
Regarding the negative charge up, Japanese Patent Laid-open No. H10-275583 has proposed a method of neutralizing an electric charge on an sample surface, said method using capillary tubes and locally supplying a gas to an observation position on the sample to cause gas molecules to crash against the sample surface, thereby ionizing the gas molecules by combining the gas molecules with electrons by the crash. However, in a mapping-projection type electron apparatus which irradiates a wide area by an electron beam, it is impossible to supply a gas uniformly to the whole portions to be irradiated by the electron beam. Consequently, the above neutralizing method is not suitable to a mapping-projection type electron apparatus.
On the other hand, regarding the positive charge up, it is contemplated that electrons are irradiated to a sample from a filament-type electron source such as Tungsten to neutralize the charge up. In this case, there is such a problem that an insulating material tends to move from a positively charged condition to a negatively charged condition, thereby going to a further negatively charged condition, which makes a control difficult. A method of reducing charge up by supplying a gas uniformly to a surface of a sample has also been proposed. Usually, however, pressure on the surface of the sample has such a large value as 0.01-0.1 Pa, so aberration occurs when an electron beam is irradiated to the sample surface and an image for observation and estimation is blurred. Consequently, this method is not suitable to a sample having a line width less than 0.1μ. In addition, since the pressure within a chamber becomes as high as 0.001-0.1 Pa, there is such a problem that an inner surface of the chamber becomes dirty, resulting in the generation of discharge at portions to which a high voltage is applied.
Furthermore, Japanese Patent Laid-open No. 2003-331774 has proposed a method of reducing charge up by irradiating a sample surface by a laser beam. However, since the laser beam is required to have an irradiation intensity of about 10 W/cm2, a great amount of energy is consumed and is not economical.